I'd like to review the activity history for one of my servers but no matter how long I leave the window it will never display the activity history for one my of sql servers. I have already checked the log activity directory and cleaned it out. Any suggestions?

What version of the tool are you on?
Can you try updating to the latest version of the tool?
If you continue to experience the issue.
The first thing to try is to clear the Activity Cache or *.dat files as follows:

1. Close the GUI connections.
2. Navigate to this directory on the machine where the SQL backup GUI is installed:
C:\\Users\\<username>\\AppData\\Local\\Red gate\\SQL Backup\\ServerData
3. In this folder you will find one or more .dat files, for example 1.dat, 31.dat, localDataCache.dat.
4. Delete all the *.dat files in the ServerData folder.
5. Now re-open the GUI. The *.dat files deleted in the previous step will be recreated. This may take awhile while the files are rebuilt. Are you now able to view the Activity History?

For a majority of customers the above resolves the issue.

However there are tiny few whose Local Data Store corrupts for some reason, so need to follow the steps below. These include repeating the above plus more.

1. Close the SQL Backup GUI
2. Navigate to this directory on the machine where the SQL backup GUI is installed:
C:\\Users\\<username>\\AppData\\Local\\Red gate\\SQL Backup\\ServerData
3. In this folder you will find one or more .dat files, for example 1.dat, 31.dat, localDataCache.dat.
4. Delete all the *.dat files in the ServerData folder.
5. On the machine where the SQL Instance is located stop the SQL Backup Agent service.
6. Locate the SQL BackupLocal Data Store. The default path is as follows:
C:\\ProgramData\\Red Gate\\SQL Backup\\Data\\<SQL Instance name>
7. Rename the existing data.sdf file to OLDdata.sdf.
8. Restart the SQL Backup Agent Service, this step will create a new data.sdf.
9. Now re-open the GUI. The *.dat files deleted in step 4 will be recreated. Are you now able to view the Activity History? Depending how much activity history there is you may need to wait for a while.